1. Field of the Invention
The invention relates in general to engraving with an electron beam wherein depressions or pits of different sizes are formed in the surface of a workpiece with an electron beam and in which the pits or depressions of different sizes are generated by changing the electron beam.
2. Description of the Prior Art
U.S. Pat. No. 4,028,523 discloses an engraving method and device for implementing the method in which cut-shaped depressions or pits are formed in the surface of a workpiece which could be a gravure or cylinder for the manufacture of a gravure printing form. The size of the cups or pits which are generated is determined by the time of incidence of the electron beam on the surface of the workpiece such that shorter incidence makes smaller pits and longer incidence make larger pits. In this system, the electron beam does not strike the surface of the workpiece in those time periods in which no processing or cutting is desired, but the beam is laterally deflected into a sump at a suitable location within the beam generator. This occurs with the use of an additional deflection arrangement which may be electrostatic or electromagnetic in operation so as to obtain a blanking effect. The variation in the size of the cups or pits determines the cup volume which is a measure of the amount of ink to be transferred by the form during the printing process can also be caused by varying the intensity of the beam. Alternatively, the duration time of the beam on the cutting surface can be varied to vary the size of the cup or alternatively, the focusing of the beam can be adjusted so as to vary the size of the cup. In U.S. Pat. No. 4,028,523 the focusing is controlled such that during the on time of the electron beam the focal plane of the beam lies closer to the surface for small cups than it does for large cups.
The engraving systems of the prior art, however, have not in practice proved practical even though they theoretically allow significantly higher work speeds due to the inertia-free control of the electron beam but conventional engraving methods such as electromagnetically driven engraving tools or traditional etching methods have generally been utilized.
One of the reasons for this is that the electron beam engraving at the beginning and during pauses in the engraving results which noticeably effect the cup geometry such as the shape and volume of the cups particularly at high speed in high frequency cutting. In other words, with a short engraving time for each cup and using high beam intensity incorrect cups result particularly at the beginning of the engraving or even at later touchup engraving such that the entire form becomes unusable.
When the electron beam is switched on or when it is again switched on after being off and during the continuing deflection for blanking between the engraving of the individual cups, a positive space charge is built up within the volume of the beam generator during the ionization of the residual gas and the positive space charge has an unfavorable influence on the static focusing of the electron beam so as to make it uncontrollable. Until a static state has occurred, a certain time passes during which the cup deformities mentioned above occur, but this cannot be accepted for the manufacture of fault-free printing forms.
Previously this effect did not have a negative influence in material processing with electron beams and in electron beam welding devices. In the manufacture of forms for printing press technology, however, the finest and most accurate structures must be produced at very high speed so this effect becomes noticeable and renders the method unusable since the eye when observing a printed product can easily detect errors which were caused by fault-free forms resulting from incorrectly engraved cups.